Inhibition of retinoic acid synthesis and its implications in fetal alcohol syndrome

Retinoic acid prevents synaptic deficiencies induced by alcohol exposure during gastrulation in zebrafish embryos

In this study, we examined the effects of alcohol exposure during gastrulation on zebrafish embryos, specifically focusing on excitatory synaptic activity associated with neurons (Mauthner cells) that are born during gastrulation. Furthermore, we determined whether co-treatment of alcohol and retinoic acid (RA) could prevent the effects of alcohol exposure during gastrulation. We exposed zebrafish embryos to ethanol (150mM), RA (1nM), or a combination of RA (1nM) plus ethanol (150mM) for 5.5h from 5.25h post fertilization (hpf) to 10.75 hpf (gastrulation). Ethanol treatment resulted in altered hatching rates, survivability and body lengths. Immunohistochemical analysis of Mauthner cells (M-cells) suggested that ethanol treatment resulted in smaller M-cell bodies and thinner axons, while electrophysiological recordings of AMPA miniature excitatory postsynaptic currents (mEPSCs) associated with M-cells showed that ethanol treated animals had a significantly reduced mEPSC frequency. Other mEPSC parameters such as amplitude, rise times and decay kinetics were not altered by exposure to alcohol. Locomotor studies showed that ethanol treatment resulted in altered C-bend escape responses. For instance, the C-bends of alcohol-treated fish were larger than control embryos. Thus, ethanol treatment during gastrulation altered a range of features in embryonic zebrafish. Importantly, co-treatment with RA prevented all of the effects of ethanol including survivability, body length, M-cell morphology, AMPA mEPSC frequency and escape response movements. Together these findings show that ethanol exposure during the brief period of gastrulation has a significant effect on neuronal morphology and activity, and that this can be prevented with RA co-treatment.

Hippocampal neuron populations are reduced in vervet monkeys with fetal alcohol exposure

Prenatal exposure to beverage alcohol is a major cause of mild mental retardation and developmental delay. In nonendangered alcohol-preferring vervet monkeys, we modeled the most common nondysmorphic form of fetal alcohol syndrome disorder with voluntary drinking during the third trimester of pregnancy. Here, we report significant numerical reductions in the principal hippocampal neurons of fetal alcohol-exposed (FAE) offspring, as compared to age-matched, similarly housed conspecifics with isocaloric sucrose exposure. These deficits, particularly marked in CA1 and CA3, are present neonatally and persist through infancy (5 months) and juvenile (2 years) stages. Although the volumes of hippocampal subdivisions in FAE animals are not atypical at birth, by age 2, they are only 65-70% of those estimated in age-matched controls. These data suggest that moderate, naturalistic alcohol consumption during late pregnancy results in a stable loss of hippocampal neurons and a progressive reduction of hippocampal volume.

Exogenous retinoic acid induces digit reduction in opossums (Monodelphis domestica) by disrupting cell death and proliferation, and apical ectodermal ridge and zone of polarizing activity function

BACKGROUND

Retinoic acid (RA) is a vitamin A derivative. Exposure to exogenous RA generates congenital limb malformations (CLMs) in species from frogs to humans. These CLMs include but are not limited to oligodactyly and long-bone hypoplasia. The processes by which exogenous RA induces CLMs in mammals have been best studied in mouse, but as of yet remain unresolved.

METHODS

We investigated the impact of exogenous RA on the cellular and molecular development of the limbs of a nonrodent model mammal, the opossum Monodelphis domestica. Opossums exposed to exogenous retinoic acid display CLMs including oligodactly, and results are consistent with opossum development being more susceptible to RA-induced disruptions than mouse development.

RESULTS

Exposure of developing opossums to exogenous RA leads to an increase in cell death in the limb mesenchyme that is most pronounced in the zone of polarizing activity, and a reduction in cell proliferation throughout the limb mesenchyme. Exogenous RA also disrupts the expression of Shh in the zone of polarizing activity, and Fgf8 in the apical ectodermal ridge, and other genes with roles in the regulation of limb development and cell death.

CONCLUSION

Results are consistent with RA inducing CLMs in opossum limbs by disrupting the functions of the apical ectodermal ridge and zone of polarizing activity, and driving an increase in cell death and reduction of cell proliferation in the mesenchyme of the developing limb.

Fetal Alcohol Spectrum Disorder (FASD) Associated Neural Defects: Complex Mechanisms and Potential Therapeutic Targets

Fetal alcohol spectrum disorder (FASD), caused by prenatal alcohol exposure, can result in craniofacial dysmorphism, cognitive impairment, sensory and motor disabilities among other defects. FASD incidences are as high as 2% to 5 % children born in the US, and prevalence is higher in low socioeconomic populations. Despite various mechanisms being proposed to explain the etiology of FASD, the molecular targets of ethanol toxicity during development are unknown. Proposed mechanisms include cell death, cell signaling defects and gene expression changes. More recently, the involvement of several other molecular pathways was explored, including non-coding RNA, epigenetic changes and specific vitamin deficiencies. These various pathways may interact, producing a wide spectrum of consequences. Detailed understanding of these various pathways and their interactions will facilitate the therapeutic target identification, leading to new clinical intervention, which may reduce the incidence and severity of these highly prevalent preventable birth defects. This review discusses manifestations of alcohol exposure on the developing central nervous system, including the neural crest cells and sensory neural placodes, focusing on molecular neurodevelopmental pathways as possible therapeutic targets for prevention or protection.

Ethanol increases retinoic acid production in cerebellar astrocytes and in cerebellum

Several characteristics of fetal alcohol syndrome (FAS) are similar to the teratogenic effects of retinoic acid (RA) exposure. It has been suggested that FAS may result from ethanol-induced alteration in endogenous RA synthesis, leading to abnormal embryonic concentrations of this morphogen. We examined whether ethanol may interfere with RA synthesis in the postnatal cerebellum, as a region of the developing CNS particularly vulnerable to both ethanol and RA teratogenesis. It was found that astrocytes are the predominant source of postnatal RA synthesis in the cerebellum. They express both retinaldehyde dehydrogenase 1 and 2. In vitro cytosolic preparations of astrocytes, as well as live cell preparations, have an increased capacity to synthesize RA in the presence of ethanol. A mechanism by which ethanol could stimulate RA synthesis is via the ethanol-activated short-chain retinol dehydrogenases, which we show to be present in the postnatal cerebellum. To determine whether ethanol stimulated RA synthesis in vivo, a sensitive and highly specific HPLC/MSn technique was used to measure cerebellar RA after administration of ethanol to postnatal day 4 rat pups. Cerebellar RA levels climbed significantly after such treatment. These results suggest that the cerebellar pathology exerted by ethanol may occur, at least in part, through increased production of RA.

Effects of prenatal alcohol exposure on forepaw digit length and digit ratios in rats

Prenatal exposure to alcohol can cause limb and digit defects. Variations in digit ratios in humans are associated with prenatal testosterone exposure. Since prenatal alcohol can reduce testosterone in rats, the effects of prenatal alcohol were measured on rat digit length. Pregnant Sprague-Dawley rats were intubated with 0 g/kg, 4 g/kg, or 6 g/kg of ethanol from Gestational Day 8 (GD8) to GD20. The 0-g/kg group and a nonintubated group served as controls. At postnatal day 31, forepaw digit lengths were measured and digit ratios calculated. Females had smaller digits on both forepaws and higher digit ratios on the right forepaw than males. Rats exposed to 6 g/kg of ethanol had smaller digits than controls on both forepaws and higher digit ratios than controls on the left forepaw. Rat digit ratios differ between the sexes, and prenatal alcohol exposure affects digit ratios. The results are consistent with a perinatal disruption of testosterone levels by alcohol and/or of testosterone's effects on digit length and ratios. An alternate interpretation is consistent with a retinoic acid-mediated effect of alcohol on digit length and ratios.

Morphogenesis and dysmorphogenesis of the appendicular skeleton

Cartilage patterning and differentiation are prerequisites for skeletal development through endochondral ossification (EO). Multipotential mesenchymal cells undergo a complex process of cell fate determination to become chondroprogenitors and eventually differentiate into chondrocytes. These developmental processes require the orchestration of cell-cell and cell-matrix interactions. In this review, we present limb bud development as a model for cartilage patterning and differentiation. We summarize the molecular and cellular events and signaling pathways for axis patterning, cell condensation, cell fate determination, digit formation, interdigital apoptosis, EO, and joint formation. The interconnected nature of these pathways underscores the effects of genetic and teratogenic perturbations that result in skeletal birth defects. The topics reviewed also include limb dysmorphogenesis as a result of genetic disorders and environmental factors, including FGFR, GLI3, GDF5/CDMP1, Sox9, and Cbfa1 mutations, as well as thalidomide- and alcohol-induced malformations. Understanding the complex interactions involved in cartilage development and EO provides insight into mechanisms underlying the biology of normal cartilage, congenital disorders, and pathologic adult cartilage.

Effect of maternal methionine pre-treatment on alcohol-induced exencephaly and axial skeletal dysmorphogenesis in mouse fetuses

Alcohol is known to induce folate deficiency and impair methionine synthase activity. Exogenous folic acid (FA) administered periconceptionally has been shown to prevent the first occurrence and recurrence of neural tube defects (NTD) in humans. Since folate, vitamin B(12) and methionine are metabolically interrelated, it was decided to determine the effect of methionine pre-treatment on alcohol-induced NTD and axial skeletal defects in mouse embryos. Following administration of a single dose of 70 or 150 mg/kg of methionine, 0.03 ml/g body weight of ethanol solution (25% v/v of absolute alcohol in saline) was injected intraperitoneally into pregnant mice at critical stages of neural tube development. The controls were either non-treated or saline treated and pair-fed and pair-watered. Fetuses were collected on gestation day 18. Alcohol and methionine plus alcohol numerically enhanced embryonic resorption and induced a significant reduction in fetal body weight. Alcohol alone caused a 3-fold increase in the background frequency of exencephaly in gestation days 7 and 8 treatment groups. The low dose of methionine only numerically reduced the spontaneous exencephaly. Pre-treatment with methionine only produced a numerical but not statistically significant reduction in alcohol-induced exencephaly. The higher dose of methionine did not produce a particularly beneficial effect on embryonic survival, fetal body weight and occurrence of exencephaly. Alcohol-induced cleft palate and limb malformations were ameliorated by methionine pre-treatment. Craniofacial skeleton, vertebrae and ribs were extensively malformed both in the alcohol and methionine plus alcohol groups indicating a lack of rescue effects of methionine. Whereas supernumerary ribs and extra sternal ribs were augmented by methionine, occipitalization of the atlas vertebra was a malformation unique to the pre-treatment group. Plasma levels of several amino acids including that of methionine were significantly lowered by alcohol. Pre-treatment with methionine produced a dose dependent enhancement of only methionine concentration. These data suggest that pre-administration of methionine only rescues mouse embryos of certain non-neural malformations and that the lack of ameliorative effect on NTD and axial skeletal defects may be due to the fact that alcohol lowers the concentration of a number of amino acids and therefore, supplementation should comprise a mixture of these amino acids and possibly FA and vitamin B(12).

Inhibition of human prenatal biosynthesis of all-trans-retinoic acid by ethanol, ethanol metabolites, and products of lipid peroxidation reactions: a possible role for CYP2E1

Biotransformation of all-trans-retinol (t-ROH) and all-trans-retinal (t-RAL) to all-trans-retinoic acid (t-RA) in human prenatal hepatic tissues (53-84 gestational days) was investigated with HPLC using human adult hepatic tissues as positive controls. Catalysis of the biotransformation of t-ROH by prenatal human cytosolic fractions resulted in accumulation of t-RAL with minimal t-RA. Oxidations of t-ROH catalyzed by prenatal cytosol were supported by both NAD+ and NADP+, although NAD+ was a much better cofactor. In contrast, catalysis of the oxidation of t-RAL to t-RA appeared to be solely NAD+ dependent. Substrate Km values for conversions of t-ROH to t-RAL and of t-RAL to t-RA were 82.4 and 65.8 microM, respectively. At concentrations of 10 and 90 mM, ethanol inhibited the conversion of t-ROH to t-RAL by 25 and 43%, respectively, but did not inhibit the conversion of t-RAL to t-RA significantly. In contrast, acetaldehyde reduced the conversion of t-RAL to t-RA by 25 and 87% at 0.1 and 10 mM respective concentrations. Several alcohols and aldehydes known to be generated from lipid peroxides also exhibited significant inhibition of t-RA biosynthesis in human prenatal hepatic tissues. Among the compounds tested, 4-hydroxy-2-nonenal (4-HNE) was highly effective in inhibiting the conversion of t-RAL to t-RA. A 20% inhibition was observed at a concentration of only 0.001 mM, and nearly complete inhibition was produced at 0.1 mM. Human fetal and embryonic hepatic tissues each exhibited significant CYP2E1 expression as assessed with chlorzoxazone 6-hydroxylation, a highly sensitive western blotting technique, and reverse transcriptase-polymerase chain reaction (PCR) (RT-PCR), suggesting that lipid peroxidation can be initiated via CYP2E1-catalyzed ethanol oxidation in human embryonic hepatic tissues. In summary, these studies suggest that ethanol may affect the biosynthesis of t-RA in human prenatal hepatic tissues directly and indirectly. Ethanol and its major oxidative metabolite, acetaldehyde, both inhibit the generation of t-RA. Concurrently, the CYP2E1-catalyzed oxidation of ethanol can initiate lipid peroxidation via generation of a variety of free radicals. The lipid peroxides thereby generated could then be further converted via CYP2E1-catalyzed reactions to alcohols and aldehydes, including 4-HNE, that act as potent inhibitors of t-RA synthesis.

Alcohol stimulates ACTH secretion in the rat: mechanisms of action and interactions with other stimuli

This review discusses some of the mechanisms through which alcohol (EtOH) alters the activity of the hypothalamic-pituitary-adrenal (HPA) axis. In adult rats, acute EtOH treatment increases plasma ACTH and corticosteroids levels primarily by stimulating the release of corticotropin-releasing factor (CRF) and possibly vasopressin (VP) from nerve terminals in the median eminence. Increased CRF gene transcription in the hypothalamus may also be important. The HPA axis remains activated during chronic EtOH exposure, although habituation may take place. Changes in the responsiveness of hypothalamic neurons, a phenomenon itself dependent in part on a number of intermediate secretagogues, as well as decreased pituitary responsiveness to VP, all play a role. Finally, the activity of the HPA axis is influenced by exposure to EtOH during embryonic development, with mature offspring showing hyporesponsiveness to many stimuli. These altered responses appear to be caused in part by changes in the synthesis/release CRF, possibly under the influence of nitric oxide. CRF, VP, ACTH, and corticosteroids are important regulators of the immune system, behavior, metabolic pathways, and reproductive parameters. Alcohol therefore may influence such functions through the pathological secretion of these hormones. A better understanding of the mechanisms through which the drug alters their release thus may permit the development of therapies designed to alleviate some of the consequences of alcoholism.

Retinoic acid concentrations in patients with squamous cell carcinoma of the head and neck

The serum concentrations of all-trans (atRA) and 13-cis (13cRA) retinoic acid were determined by high performance liquid chromatography in 27 patients with squamous cell carcinoma of the head and neck and in 80 healthy controls. This investigation seemed relevant as ethanol is an aetiological factor in these cancers and has been suggested to interfere with the synthesis of atRA. Neither the serum concentration of atRA nor that of 13cRA differed between patients and controls. The serum atRA concentration did not differ between fasting and non-fasting patients, but the serum 13cRA concentration was significantly higher in non-fasting than in fasting patients, probably due to the dietary retinoid content.

Low serum concentration of all-trans and 13-cis retinoic acids in patients treated with phenytoin, carbamazepine and valproate. Possible relation to teratogenicity

All-trans retinoic acid deficiency resulting from ethanol's interference with the synthesis of all-trans retinoic acid from retinol was recently suggested to cause the malformations of the fetal alcohol syndrome. Phenytoin, carbamazepine and valproate, might be teratogenic because they lower the concentration of all-trans retinoic acid in serum, by inducing the enzyme systems in the liver responsible for the metabolism of the all-trans retinoic acid, or by other mechanisms. Here we show, that in patients given therapeutic doses of phenytoin, carbamazepine and valproate, serum all-trans and 13-cis retinoic acid concentrations are indeed significantly lowered. We propose that drugs with this ability should be considered as potential teratogens.

Maternal risk factors in fetal alcohol syndrome: provocative and permissive influences

We present an hypothesis integrating epidemiological, clinical case, and basic biomedical research to explain why only relatively few women who drink alcohol during pregnancy give birth to children with alcohol-related birth defects (ARBDs), in particular, Fetal Alcohol Syndrome (FAS). We argue that specific sociobehavioral risk factors, e.g., low socioeconomic status, are permissive for FAS in that they provide the context for increased vulnerability. We illustrate how these permissive factors are related to biological factors, e.g., decreased antioxidant status, which in conjunction with alcohol, provoke FAS/ARBDs in vulnerable fetuses. We propose an integrative heuristic model hypothesizing that these permissive and provocative factors increase the likelihood of FAS/ARBDs because they potentiate two related mechanisms of alcohol-induced teratogenesis, specifically, maternal/fetal hypoxia and free radical formation.

Fetal alcohol syndrome: the vulnerability of the developing brain and possible mechanisms of damage

Fetal alcohol exposure has multiple deleterious effects on brain development, and represents a leading known cause of mental retardation. This review of the effects of alcohol exposure on the developing brain evaluates results from human, animal and in vitro studies, but focuses on key research issues, including possible mechanisms of damage. Factors that affect the risk and severity of fetal alcohol damage also are considered.

The consequences of prenatal substance use for the developing fetus, newborn, and young child

Although substance use has been a worldwide problem at all levels of society since ancient times, recent attention has been focused on the use of legal and illegal substances by the pregnant woman. Almost all drugs taken by the pregnant woman are known to cross the placenta and have some effect on the fetus. This article reviews the effects of the drugs most frequently used by pregnant women in the United States--nicotine, alcohol, marijuana, opiates, and cocaine--on the fetus and neonate; when possible, information regarding long-term medical problems is included.

Chronic ethanol administration enhances retinoic acid and triiodothyronine receptor expression in mouse liver

Chronic alcoholism induces perturbations of storage and metabolization of retinol and related compounds. After 6 months of ethanol consumption we have observed in mouse liver an increased expression of Tri-iodothyronine receptors (TR) while the expression of retinoic acid (RA) receptors (RAR) was unaffected. After 10 months of alcoholization the TR expression was strongly increased and the RAR expression was also increased. At this time the activity of aldehyde dehydrogenase and that of alcohol dehydrogenase, two enzymes involved in biosynthesis of RA from retinol, were similar in the liver of alcoholized and pair-fed mice. Thus it can be hypothesized that (i) the change of RAR expression was, at least in part, the result of a change of TR expression (result in agreement with previous data), (ii) the increased expression of RAR could induce apoptosis and subsequently liver necrosis.

Maternal ethanol ingestion effects on fetal rat brain vitamin A as a model for fetal alcohol syndrome

Fetal embryo, head, and brain tissue from different gestational ages were analyzed for retinol content, nuclear retinoic acid receptor and cytosolic retinoic acid binding protein levels after maternal ethanol ingestion and compared with fetal levels in control diet pregnancies. Retinol levels in fetal embryo and brain of ethanol-ingesting pregnancies were 2- to 3-fold higher than fetal embryo and brain retinol of control pregnancies. Nuclear retinoic acid receptor was lower in 10-day embryo of ethanol pregnancies and apparently unaffected in fetal head and brain by maternal ethanol consumption at other days of gestation. In fetal head there was a significant overall ethanol effect on cytosolic retinoic acid binding protein, with increased levels in fetal tissue from ethanol-consuming pregnancies. These observations of altered embryo, fetal head, and fetal brain retinol and receptor protein levels support the hypothesis of a possible role of vitamin A in fetal alcohol syndrome.

Rat brain microsome fluidity as modified by prenatal ethanol administration

The fluorescence anisotropy (r) of diphenylhexatriene (DPH) and of trimethylamino-diphenylhexatriene (TMA-DPH) as a function of temperature (10 degrees to 54 degrees C) was measured in brain microsomes of newborn rats prenatally exposed to ethanol. In this temperature range, the relationship between r and T was linear. The addition of ethanol in vitro to microsomal suspensions influenced the slope of the line of r versus T only when DPH was used as a probe and with high concentrations of the alcohol (> or = 0.3 M). The administration of ethanol (18% of total energy intake) in vivo to pregnant dams affected the slope of the lines of r versus T of the microsomes of pups, either using DPH or TMA-DPH as probes. The slope was also affected in brain microsomes obtained from dams, yet, only with TMA-DPH and in the opposite sense than in pups. We conclude that the effect of prenatal exposure to ethanol depended on metabolic alterations induced by the alcohol and not on its detergent properties for the following reasons: (a) The effects in vitro and in vivo were different and (b) in vitro effects could be obtained only with high concentrations (> or = 0.3 M), whereas in vivo effects were produced by small doses of ethanol. Besides, the effects of the administration of the alcohol in vivo were different in adult and intrauterine life.

Ethanol inhibits C6 cell growth: fetal alcohol syndrome model

Maternal consumption of ethanol produces a pattern of malformations, including nervous system abnormalities, in the developing fetus, a state called Fetal Alcohol Syndrome. We report the dose-dependent inhibition by ethanol of the growth of a glioma derived cell line, C6 cells; the effects occur at ethanol concentrations commonly encountered in the blood during human intoxication. The effects occur with different morphological subtypes of the cell line and do not occur when the cells are exposed to iso-osmolar concentrations of other chemicals. The results demonstrate that C6 cells are a model for the study of the effects of ethanol on nervous system cell growth.